1. Field of the Invention
The present invention relates to a carrier for use in a developer for developing an electrostatic latent image formed by an electrophotographic method, an electrostatic printing method, an electrostatic recording method or the like method. In addition, the present invention relates to a method for manufacturing the carrier.
2. Discussion of the Background
Two component developers which are constituted of a carrier and a toner are widely used for developing an electrostatic latent image formed on an image bearing member by an electrophotographic method, an electrostatic printing method, an electrostatic recording method or the like method. In particular, in recent years resin-coated carriers in which a core material is coated with a resin are typically used to enhance the durability of the carrier. Such resin-coated carriers have a good electric insulation property, and therefore, the resin-coated carriers have a good charge imparting property. In addition, the surface of the resin-coated carriers have good smoothness, and therefore the carriers have good durability.
However, the resin-coated carriers have a drawback in that the carriers produce images having a poor solid image in which only the edge portions of the solid image are emphasized and the other portions thereof has poor image density (hereinafter this phenomenon is referred to as an edge-emphasized image). In attempting to remedy this drawback, a half-coated carrier in which a part of the surface of carrier particles is coated with a resin (i.e., projected portions of the core material are exposed without being coated) has been proposed in Japanese Laid-Open Patent Publications Nos. 3-160463 and 4-93954. Since in the half-coated carrier project portions of the core material are exposed without being coated, the half-coated carrier has relatively low electric resistance. Therefore, the charges tend to be easily transferred, resulting in formation of a solid image having high image density (without causing an edge-emphasized image). However, since the half-coated carrier has relatively low electric resistance, a charge whose polarity is opposite to that formed on an image bearing member is injected to the carrier, resulting in occurrence of a carrier adhesion problem in that the carrier itself adheres to the image bearing member.
In addition, in recent years a toner recycle system is used in image forming apparatus in which toner particles, which are collected in the image forming apparatus without being used for developing electrostatic latent images, are reused to protect environment. Namely, the toner recycle system is such that toner particles remaining on an image bearing member without being transferred to a receiving paper are collected by a cleaning device, and the collected toner particles are supplied again to the developing area or the toner supplying device to be reused. In the collected toner particles, external additives of the toner particles tend to be embedded into the toner particles, and therefore the surface of the carrier is hardly abraded by the toner. Therefore the surface of the carrier is hardly refreshed, i.e., the toner tends to accumulate on the surface of the carrier, resulting in occurrence of a spent-toner problem. When the spent-toner problem occurs, the charge quantity of the developer deteriorates, resulting in occurrence of background fouling in the resultant toner images. Therefore, the developer has a short life. In particular, the half-coated carrier has projected portions which are not coated with a resin, and therefore this spent-toner problem easily occurs.
Because of these reasons, a need exists for a developer having good durability and capable of producing images having good image qualities even when used for an image forming apparatus using a toner recycle system for a long time.